1. Field of the Invention
The present invention relates generally to ground-based dynamic testing of low-frequency structures and more particularly to a suspension device for testing such structures.
2. Description of the Related Art
Space structures in general experience free-free boundary conditions that are not readily replicable in ground based dynamic testing. Numerous devices have been proposed to conduct dynamic tests while supporting the weight of the structure without introducing any constraining forces which can impose boundary conditions not found in space. For example, long cables have been used to suspend the structures from a high ceiling. Dynamics testing is conducted in the horizontal plane to reduce any gravitational effects on the structural dynamics. However, this suspension gives rise to an undesired pendulum effect as the induced horizontal motion combines with the vertical cable support. In addition, this proposal has the additional drawback of large space requirements for the high ceilings and the introduction of undesired low frequency vibrations to the structures during movement due to elasticity of the long cables.
Various other designs are based on the principle of reducing or eliminating friction in the horizontal plane. The most common design involves air pads which act as hydrostatic air bearings to suspend the structure. Once again the testing is performed in the horizontal plane. These air bags are incorporated into the structure and may have a mass sufficient to distort the actual dynamic characteristics of the structure. In addition, this proposal requires a large air table as well as air pumps, filters and other pneumatic equipment to reduce the friction between the air pads and the air table.
A pneumatic electric device is also used comprising an external air tank under pressure for driving a piston supporting the test structure. Since the pneumatic system incurs a positive spring stiffness, a linear D.C. motor is incorporated to introduce a negative spring stiffness to the pneumatic system to create a perspective of a low stiffness toward the structure. This requires a very complex control system with closed loop feedback to ensure proper operation and stiffness compensation.
Finally, various spring configurations have been proposed which attempt to introduce a near zero stiffness of the suspension system. These configurations only result in a very small domain of operation, i.e., a very small stroke, through which the test structure can move without any constraining force.